The present invention relates to a method and apparatus for accomplishing in situ vitrification of soil and particularly to such a method and apparatus as promotes cold cap subsidence.
The presence of waste or contaminated material buried in the ground is a subject of concern relative to dispersion of the waste and contamination of the surrounding environment. Various methods have been employed for stabilizing or solidifying waste sites, for example the injection of soil reactant chemicals or solidifying compositions of material such as concrete. Stabilization has also been accomplished by artificially freezing the soil.
A particularly attractive method for solidifying or destroying hazardous waste material in situ and for general soil stabilization is described and claimed in Brouns et al. U.S. Pat. No. 4,376,598 entitled "In Situ Vitrification of Soil", issued Mar. 15, 1983. According to this method, two or more electrodes are placed in the ground and after a start-up procedure a substantial current is passed therebetween resulting in liquification or melting of the soil. The waste materials will be melted, pyrolized or dissolved in the molten soil. A gas collection apparatus is placed over the site for removing gaseous products of combustion. After melting, the current flow is discontinued whereupon the theretofore liquified mass hardens into vitrified material comprising for the most part a glass, a glass ceramic or a partially devitrified glass, having crystals and glass dispersed in a solid matrix. Any metals within the mass may be dissolved in the glass especially as oxides, or may form solids at its lower boundaries.
According to this procedure, the soil undergoing treatment reduces somewhat in bulk and "subsides" leaving a depressed area or pocket which is then backfilled to the original ground surface, further burying the solidified waste material. This mode of operation is achieved, for example, when employing electrodes exhibiting high electrical and heat conductivity such as graphite electrodes. Unfortunately, graphite electrodes are also short lived since they oxidize readily above the level of the molten surface. A graphite electrode life in excess of forty hours has not been demonstrated under these conditions. Graphite electrode failure occurs from oxidation at the molten soil surface at temperatures in excess of 600.degree. C. Electrodes formed of molybdenum have heretofore been preferred.
Unfortunately, molybdenum electrodes of practical size do not appear to promote subsidence of the soil surface as hereinabove described, but rather tend toward causing the formation of a "cold cap", i.e., bringing porous vitrified soil to the surface and even above the original soil surface. This cold cap can create a disposal problem and/or require extensive backfill. Moreover, the porous cold cap may be a subject of concern regarding possible future compaction of the porous vitrified material.
In any case, electrode life in the above processes is shorter than desired because of the high operating temperatures and high electrode oxidation rates encountered. Replacement of electrodes during processing after failure can represent a major expenditure in waste control by vitrification.